Asphalt is a black sticky, semi-solid material with high viscosity at room temperature. Asphalt is classified into several grades, depending on its hardness, for example, penetration, viscosity and compatibility.
Asphalt is in general graded through penetration grade test according to ASTM D946. The penetration grade is an index showing the hardness of asphalt and is determined by penetrating a standard probe into asphalt at a given temperature, specifically at 25° C., under predetermined load and time. It is measured as the depth of the probe penetrating the asphalt in 0.1 mm unit when the probe is pressed with a force of 100 g for 5 seconds. In general, the smaller the value, the harder the asphalt. There are five standard penetration grades: 40-50, 60-70, 85-100, 120-150 and 200-300. For example, the asphalt with penetration grade of 40-50 is harder than the penetration that with grade penetration grade of 200-300. The two typical types of asphalt for road pavement produced in Korea are AP-3 with penetration grade of 85-100 and AP-5 with penetration grade of 60-70.
When asphalt is exposed to a high temperature condition, it becomes viscous, thereby resulting in plastic deformation. On the contrary, when it is exposed to a low temperature condition, it becomes brittle upon external impact.
Typically, additives such as SBS block copolymer, polyolefin resin, styrene-butadiene rubber, and the like are used to reduce temperature susceptibility of asphalt. The asphalt having improved properties rendered by the addition of additives is called “modified asphalt.” Among these additives, SBS block copolymer is most widely used [J. of Materials Science, 36, 2001, p. 451-460].
With respect to the physical properties of modified asphalt using SBS block copolymer, the melting rate of the SBS block copolymer in asphalt serves as an important factor.
The mechanism by which the SBS block copolymer is melted in asphalt is as follows [see FIG. 1(a)]. First, the modifier SBS block copolymer is added to asphalt at 160-200° C. (stage 1). Then, the SBS block copolymer is mixed with the asphalt (stage 2), and an aromatic oil component of the asphalt penetrates into the SBS block copolymer (stage 3). Subsequently, the SBS block copolymer swells and is dispersed in the asphalt to form a physical network (stage 4) [J. of Materials Science 36, 2001, p. 451-460]. From the above, it is known that the melting rate of the SBS block copolymer in the asphalt depends on how fast the SBS block copolymer becomes swollen by the asphalt component.
The asphalt modified using the SBS block copolymer is widely used as modified asphalt for road pavement, waterproof sheets, etc.
The modified asphalt for road pavement is prepared by adding and melting 3-10 weight % of a pellet or crumb type modifier (e.g., SBS block copolymer) in asphalt in a vast melting facility and mixing the modified asphalt with aggregate in an ascon plant (premix type wet method), or by adding 5-15 weight % of a modifier to asphalt when mixing the asphalt with aggregate in an ascon plant to prepare ascon (plant mix type dry method).
Of the two techniques for preparing a modified asphalt for road pavement, the premix type is more advantageous in that large-scale production is possible because the modifier is melted and mixed in asphalt at 160-200° C. in a vast melting facility, and is thus widely used for road pavement. However, in case the modified asphalt should be transported over a long distance from the melting facility to an ascon plant, the modified asphalt may experience deterioration in properties because of aging during the lengthy transport. Further, this technique is not suitable for the pavement of a short road section requiring only a small amount of modified asphalt. In addition, phase separation between the modifier and the asphalt may occur during the storage or transport.
The plant mix type is advantageous in that the phase separation problem and the deterioration of physical properties at high temperature during the manufacture and transportation may be minimized because the modifier is mixed with asphalt in an ascon plant and without the storage process. This method is widely used when the application of the premix type is difficult, for example, pavement of a short road section, pavement of a road with poor accessibility, maintenance of road, or pavement using special asphalt such as drainable asphalt. In the plant mix type, it is important that the asphalt modifier is quickly melted in the asphalt. Retardation in melting may result in retarded preparation of asphalt mixture or the modified asphalt may not exert desired property.
To solve this problem, Korean Patent Nos. 417,294 and 655,635 disclose a modifier composition for improving the melting rate of the asphalt modifier used in the plant mix type, which comprises a mixture of SBS block copolymer, adhesion-providing resin and processing aid. However, these modifiers are also limited in melting rate when high viscosity SBS block copolymer is used to prepare the composition [Korean Patent No. 701821.
Further, the cost of the adhesion-providing resin and the processing aid is very high compared to that of asphalt, and thus the cost competitiveness is behind the premix type large-scale production. In addition, oil, which is used as the main processing aid, tends to evaporate while the ascon produced in the ascon plant is transferred to the pavement site, resulting in fuming of the ascon.
The modified asphalt for waterproof sheets is prepared similarly as for the modified asphalt for road pavement premix type. 8-20 weight % of SBS block copolymer is added and melted in a mixture of asphalt and an inorganic material in a melting facility of 160-200° C. Then, the melted mixture is passed through a cloth to prepare a waterproof sheet. At this time, if the SBS block copolymer is melted slowly, the physical properties of the final product may be deteriorated due to thermal oxidation and aging caused by extended exposure to heat [Journal of the Korean Society of Road Engineers, Vol. 6, No. 2, 2004, p. 15-24].
In order to obtain high melting rate in the premix type technique (or modified asphalt for waterproof sheet), Korean Patent Nos. 701821 and 581,820 disclose a method of improving the melting rate by making the asphalt modifier porous using a blowing agent, thereby increasing the surface area. However, the method of using a blowing agent is disadvantageous in that foaming may occur without discontinuation during the storage or use unless the blowing agent is consumed completely in the melting process, and the byproducts formed during the foaming process may result in contamination and deteriorated physical properties of the asphalt modifier.